Concrete has been, and is, one of the most widely used building materials. Concrete is a mixture of an aggregate and a binder. Portland cement is a common binder. Gravel is often used as an aggregate. The aggregate and binder are mixed together with a hydrating agent, such as water. Upon hydration the binder solidifies and binds the aggregate together into a solid mass.
Various inventors have proposed using plastic particles as aggregates to produce concrete which is less dense than concrete made using conventional gravel aggregates. For example, Raponi, U.S. Pat. No. 4,058,406 discloses a cementitious composition which contains waste polyethylene strips. Sweeney, U.S. Pat. No. 5,209,968 discloses a building panel having a core made from scrap or waste plastic and a cementitious slurry binder. The core in the Sweeney panel is sandwiched between a pair of high strength outer layers.
A problem with the use of plastic materials for aggregates in concrete compositions is that most common binders, such as portland cement, do not adhere well to common plastics. The result is a concrete material which is significantly weaker than ordinary concrete. This a particular problem in mixes which contain relatively large proportions of plastic aggregates. Concretes using plastic aggregates have generally been confined to uses in which strength is not required and to mixes having relatively low volumetric ratios of plastic aggregates.
There is a persistent need for strong low density concrete formulations that are not unduly expensive. There is also a persistent need for a way to reuse ex-consumer plastic materials.